tree-wide: Use pidref_wait_for_terminate_and_check()

Let's use the pidref variants for waiting and
checking a process everywhere and get rid of the
non pidref variants.

src/basic/namespace-util.h

@@ -82,7 +82,7 @@ int parse_userns_uid_range(const char *s, uid_t *ret_uid_shift, uid_t *ret_uid_r
 int userns_acquire_empty(void);
 int userns_acquire(const char *uid_map, const char *gid_map, bool setgroups_deny);
 int userns_acquire_self_root(void);
-int userns_enter_and_pin(int userns_fd, pid_t *ret_pid);
+int userns_enter_and_pin(int userns_fd, PidRef *ret);
 bool userns_supported(void);
 
 int userns_get_base_uid(int userns_fd, uid_t *ret_uid, gid_t *ret_gid);